The scientific study of periodic biological phenomena, such as migration and flowering, in relation to climate.
Yesterday was my first 2013 sighting of a Sandhill crane; in 2012 I'd sighted several by February 29. Last year, I first noticed a robin on March 6, and crocuses were blooming; by this time tomorrow there should be close to 9 inches of new snow on the ground, so I don't expect to see the crocuses anytime soon.
Those are just one person's observations over a year, but scientific study of periodic phenomena requires large amounts of data spanning as much time as possible. Groups such as the USA National Phenology Network rely on fleets of volunteer citizen-scientists to report on events in their area, and historical records of blossom and harvest times provide phenological data that spans centuries.
Etymology: late 1800s. From Greek phaino (to show, to make appear) + logos (study), so I guess one could roughly translate it as "the study of when things appear".
(Not to be confused with phrenology, speculations based on the shape and size of a human skull.)